1. Field of the Invention
The present invention generally relates to radio pagers, and more specifically to a multi-area radio pager operating in a battery saving mode immediately after losing contact with a communicating channel due to hindrance by a building or an underground structure.
2. Description of the Related Art
Multi-area radio display pagers have been developed to enable users to be paged in a number of service areas in which different frequencies are used. Such radio pagers are provided with a scan list memory which contains data concerning assigned frequencies and unique codes of the areas in which the pagers are entitled to be serviced. The pager is programmed to constantly monitor a signal it is receiving. Once the current field strength drops below a defined threshold, it makes a search through the scan list in an attempt to select the frequency of a new area. If the cause of the low field strength is due to the fact that the pager has moved out of a service area, the attempt is to find a new channel, and if this is caused by hindrance of the signal path by an obstacle or underground structure, the attempt is to recapture the previous channel. Such multi-area pagers operate in a battery saving mode during standby periods.
Japanese Laid-Open Patent Specification Hei-6-315001 discloses a multi-area pager in which the energy-consuming receiver is periodically turned on for a brief interval to select a frequency according to the frequency data of in the pager's scan list. The receiver is then turned off if it fails to synchronize to the selected frequency or if it is successful in the attempt but fails to detect the pager's address in the synchronized signal. This process will be repeated until the pager's address is detected in a synchronized signal.
Since the ratio of the turn-on time to the turn-off time is a measure of the efficiency of the battery saving operation, a ratio of 1/15 is usually employed. Although satisfactory when the pager is entering a new area and making a search for a new frequency, the ratio of this value is unsatisfactory when the pager is making an attempt to resynchronize to the previous frequency after it suddenly lost contact with the transmitter. However, as the number of frequencies assigned to a paging system increases, the amount of time taken to complete a scan across the assigned frequencies during such resynchronization process becomes substantial.
As a result, a serious problem can occur in a multi-area radio pager when performing repeated resynchronization attempts after it briefly lost contact with the transmitter. If a system has 40 allocated frequencies and each pager takes 2 seconds to synchronize to each frequency during a search, the turn-on time to complete a search is 80 seconds and hence the turn-off time of the pager to achieve the saving ratio of 1/15 is equal to 18 minutes 40 seconds (=80.times.15-80) as shown in FIG. 1. Therefore, in a worst case a multi-area pager would start a search through the scan list from time t.sub.1, and resynchronize to one of the allocated frequencies at time t.sub.2, taking a maximum of 20 minutes.